1. Field of the Invention
The present invention is directed to combustion of hydrocarbon fuel and in particular to a burner with a swirler body for use in hydrocarbon fueled combustion reactors.
2. Brief Description of the Related Art
Burners with a swirling flow of a combustion reactant are mainly used for firing gas-fueled industrial furnaces and process heaters, which require a stable flame with high combustion intensities. Conventionally designed swirling flow burners include a burner tube with a central tube for fuel supply surrounded by an oxidizer supply port. Intensive mixing of fuel and oxidizer in a combustion zone is achieved by passing the oxidizer through a swirler installed at the burner face on the central tube. The stream of oxidizer is, thereby, given a swirling-flow, which provides a high degree of internal and external recirculation of combustion products and thus a high combustion intensity.
As a general drawback of conventional swirling-flow burners of the above design, the burner face experiences high gas flow velocities, as required for industrial burners of this design, and is exposed to overheating caused by the high degree of internal recirculation along the central axis of the combustion zone. Hot combustion products flow, thereby, back towards the burner face, which results in rapid heating, up to high temperatures and, consequently, destruction of the face.
A swirling burner for use in small and medium scale applications with substantially reduced internal recirculation of combustion products toward the burner face is disclosed in U.S. Pat. No. 5,496,170. The burner design disclosed in this patent results in a stable flame with high combustion intensity and without detrimental internal recirculation of hot combustion products by providing the burner with a swirling-flow of oxidizer having an overall flow direction concentrated along the axis of the combustion zone and at the same time directing the fuel gas flow towards the same axis.
The disclosed swirling-flow burner comprises a burner tube and a central oxidizer supply tube concentric with and spaced from the burner tube, thereby defining an annular fuel gas channel between the tubes, the oxidizer supply tube and the fuel gas channel having separate inlet ends and separate outlet ends. U-shaped oxidizer and fuel gas injectors are arranged coaxially at the burner face. The burner is further equipped with a bluff body with static swirler blades extending inside the oxidizer injector. The swirler blades are mounted on the bluff body between their upstream end and their downstream end and extend to the surface of the oxidizer injection chamber.
In burners for large-scale reactors, the swirler blades will have an extended length and area, which decreases the mechanical stability of the blades. Alternatively, the bluff body has to be constructed with a larger size to reduce the length of the swirler blades.
Disadvantageously, the swirler blades in a large-scale swirler burner have a size, which causes mechanical stability problems and unintended vibrations. Alternatively, when upscaling the swirler bluff body, the pressure drop of oxidizer flowing around the body will disadvantageously increase. Widening the outlet end of the fuel and/or oxidizer supply tube may compensate for the increasing pressure drop. However, the desired flow pattern around the axis of the combustion zone will then be disadvantageously scattered around the axis.
Thus, the main object of the invention is to obtain a swirler body, preferably for use in large-scale swirler burners with a bluff body and swirler blades having a size and shape without the above problems in large-scale swirler burners.
Accordingly, this invention is a swirling-flow burner including a burner tube with an outer fuel supply tube and a central oxidizer supply tube concentric with the fuel supply tube. According to an alternative embodiment the swirling-flow burner with a burner tube includes a central oxidizer supply tube and an outer concentric fuel supply tube, the oxidizer supply tube being provided with a concentric cylindrical guide body having static swirler blades and a central concentric cylindrical bore, the swirler blades extending from the outer surface of the guide body to the inner surface of the oxidizer supply tube, being concentrically arranged within a space between the guide body and inner wall at a lower portion of the oxidizer supply tube.
In a further embodiment, the burner also includes the swirling-flow burner central borestatic swirler blades and a central bluff body, the static swirler blades extending from surface of the bluff body to a surface of the guide body.
Additional stabilization of the swirler blades during operation is obtained by fixing the outer swirler blades in the above burner on the inner surface of the oxidizer supply tube. The guide body is then mounted on an outer edge of the inner swirler blades.
The inner swirler blades are preferably formed by machining the outer surface of the bluff body.
The swirling-flow induced in the swirler promotes mixing of fuel gas and oxidizer by increasing the area of their contact. Effective mixing is obtained when adjusting the pitch angle of the swirler blades to an angle of between 15xc2x0 and 75xc2x0, preferably between 20xc2x0 and 45xc2x0.
An increased mixing of fuel gas and oxidizer is additionally provided, when arranging the inner swirler blades around a central part of the bluff body and an upper portion of the guide body, and the outer swirler blades around a lower portion of the bluff body and a lower portion of the guide body.
An inwardly directed flow pattern of combustion reactants along an axis of a combustion zone adjacent to the burner face is obtained by U-shaped contours of outlet ends of the fuel, and oxidizer supply tubes, and prevents recirculation of hot combustion products in a high temperature region around axis of the combustion zone, which otherwise leads to overheating and destruction of the burner face.
The inwardly directed flow pattern leads to a high degree of external recirculation in a low temperature outer region of the combustion zone. From this region only cooled combustion products flow back to the burner face, where the products are being sucked into the hot combustion zone area and reheated there.
To maintain substantially the above flow pattern it is additionally preferred that a contour of the guide body follows the contour of the inner wall of the oxidizer supply tube.
When operating a burner according to the invention in gas fired reactors, the recycle stream of cooled combustion products protects advantageously the reactor walls surrounding the combustion zone against impingement of hot combustion products and prolongs the lifetime of the reactor. The temperature at the burner face close to the outlet end of the injection chambers may further be lowered by forming the oxidizer tube at the outlet end sharp-edged with a minimum tip angle. Reduced heating and suitable mechanical strength of the injector are obtained at tip angles of between 15xc2x0 and 60xc2x0, preferably between 15xc2x0 and 40xc2x0.
As a further advantage of the burner according to the invention, the high degree of external recirculation of cooled combustion products provides a homogeneous temperature distribution in the combustion outlet zone.
This is of great importance during operation of fired catalytic reactors, where the product yield highly depends on the temperature distribution in the catalyst bed, which typically is arranged in the combustion outlet zone.
In another embodiment of the invention the guide body is solid and provided with a bore in the middle, concentric with the axis of the burner. This ensures the swirling effect as above with the same radius of outer swirler blades, and simultaneously low pressure drop when operating with high oxidizer gas flows. The hot reaction zone is forced away from the burner tip, still maintaining the rotation of the reacting gas around the axis of the burner.
Accordingly, the burner of this invention is particularly useful in largescale gas-fueled reactors with heating and catalytic processes without creating additional pressure drop or mechanical instability.
The above objects and advantages of the invention are explained in more detail in the following description by reference to the drawings, in which the figures show a cross sectional view of the lower portion of a fuel and oxidizer supply tube in a swirling flow burner according to two specific embodiments of the invention.